branch: delete obsolete message about changing branch of obsolete commit We now rely on `rewriteutil.precheck()` to check for divergence, so we don't need the extra check in `cmdutil.changebranch()`. The former check is a little less strict in that it allows you to rewrite a commit without non-obsolete successors. Differential Revision: https://phab.mercurial-scm.org/D10519